1. Field of the Invention
The present invention relates to a method and apparatus for correcting for Doppler induced error in a GSM (global system for mobile communication) handset of the dual band type that is able to communicate on both GSM and UMTS (Universal Mobile Transmission Standard) networks.
2. Description of the Prior Art
Most mobile phones in Europe now operate on the GSM transmission standard. However, the next generation of the mobile telephones and other portable communication devices will adopt the UMTS system. In the transitional period, it will be necessary for phones to be able to work on both systems and this is a situation which may prevail for some time.
In order for the phone to operate properly in both GSM and UMTS modes, the clocks for the two modes must be translatable. Thus, the handset must not lose synchronization with the GSM network when it is used on the UMTS network and vice versa.
When the phone is not in a call, this is not a problem as there is plenty of time available for the phone to perform all required synchronization measurements with both networks.
When the phone is in a call on the UMTS network, there are commands available to the phone to cause transmission gaps between the phone and a base station with which it is in communication, so again there is no particular problem in synchronizing to the UMTS clock. Here, the transmission gap is a difference of time or frequency due to disturbance such as propagation delay and fading.
However, when the phone is in a call on the GSM network, the only times available for making synchronization on clock measurements with the UMTS network are “idle” periods which are 1 GSM frame in length and approximately 120 milliseconds apart from each other. Certain of the measurements required to synchronize with UMTS take longer than 120 milliseconds to perform and so have to be split over more than one idle frame. To successfully complete such a measurement, the mobile unit has to keep track of the UMTS frame timing (which it must establish during the first GSM idle frame) until the next GSM idle frame. The accuracy required for this depends on the window size of the UMTS decoder. Since window size relates directly to the processing power required and hence to the cost and power consumption of the decoder, the window size needs to be kept as small as possible to minimize cost and power consumption. Thus, the interests of cost and power consumption work against the requirement for accurately maintaining synchronization with the UMTS clock. In such instances, the only clock available to time the UMTS operation is the GSM clock. This can apparently vary with respect to the UMTS system and the size of variation, and hence the window size required to achieve synchronization depends on two factors:
1. the difference between UMTS and GSM network clocks at the respective base stations; and
2. the Doppler shift in the GSM signals caused by the motion of the handset relative to the GSM base station. When a handset is “camped on” to a network, it automatically locks its local clock to the observed signals from the network. These signals are Doppler shifted if the handset is moving. As a UMTS base station is not necessarily at the same location as the GSM base station, the Doppler shift may cause errors in UMTS timebase calculation.